Vortex flowmeter transducer

ABSTRACT

A vortex flowmeter including a differential pressure transducer with high common mode rejection. No diaphragms are required. A relatively thick machined recess at one end of the vortex shedding body concentrates and transmits the vortex shedding differential pressure to strain transducers located outside the fluid flow conduit. Embodiments are disclosed in which piezoelectric elements and reflective optical fibers are used as strain transducers. The described arrangements allow the mechanical clamping of the transducers and their easy removal without a flow shut down. The shedder shape is optimized in relation to the differential transducer to strengthen the shed vortices and improve the linearity and repeatability of the meter.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of Ser. No. 129,123 filedDecember 4, 1987 entitled Vortex Flowmeter Transducer now U.S. Pat. No.4,864,868.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of vortex flowmeters and, moreparticularly, to a design for a differential pressure transducer for usewith a vortex flowmeter which requires no fill fluids, and whichutilizes mechanical clamping of the vortex sensing elements.

2. Description of the Prior Art

The phenomenon of vortex shedding occurs over a certain Reynolds numberrange when a fluid (gas or liquid) flows past a bluff (non-streamlined)body. In a two-dimensional flow, the vortices formed on the oppositesides of the body rotate in an opposite sense from each other and form aregular geometrical pattern called the Karman vortex street. Theconvection velocity of this geometrical pattern is directly relatedsolely to the approaching stream velocity. This means that the sheddingfrequency is proportional to the flow rate regardless of the fluidproperties. The detection of the number of vortices shed per unit time(the vortex shedding frequency) and not its strength, is the primemeasurement property of interest.

The role played by the interaction of the two shear layers on the sidesof the bluff body is central in explaining the phenomenon of thealternation of vortex shedding. This was explained by J. H. Gerrard(1966) in Journal of Fluid Mechanics, 25, 401-413. A vortex spiral formson either side of the wake of a bluff body or strut as a result of theshear layer instability. The vortex spiral continues to grow (see, forexample, FIG. 4) until it is strong enough to draw the opposing shearlayer across the near wake. The arrival of an oppositely signed vortexstarts to weaken the ability of the spiral to draw vorticity from itsconnected shear layer. This allows the growing vortex to separate andmove downstream. The bent shear layer from the other side that wassucked into the side of the spiral then begins to feed its own spiral onthe opposite side of the strut. The presence of the strut between thetwo shear layers is essential for this alternation mechanism to work.This fact emphasizes the critical role played by the extent and shape ofthe back of the strut.

Vortex shedding results in alternating pressure depression on the twosides of the strut. A number of different techniques to detect thealternation frequency have been proposed. The cooling effect of thepressure depression or of a resulting induced flow that tries to restorethe pressure balance is used in thermal sensors. Thermal sensors rely onhaving a heated element in the flow. This represents a potential hazard.The signal is often noisy and requires complicated electronicconditioning. The transducer element is delicate and subject to theaging process. The main advantage of thermal sensors is high sensitivitywith a good time response.

In another technique, shuttle elements (discs, balls, etc.) are placedin a cavity that connects the two strut sides and move due to theinduced flow that results from the pressure imbalance created by vortexshedding. Several proximity transducers to detect the movement of ashuttle element are available. The electronics are relatively simplewhen compared to thermal sensors. However, these techniques requirepassageways with openings that are susceptible to clogging and build-upof debris. The presence of a moving mechanical element limits thetransducer time response and its fatigue cycle. The passageway is aleakage passage that affects the shedding process. In light of Gerrard'smodel (see above) that explains the alternation of vortex shedding, thisleakage effect accelerates the equalization of the differentialpressure, i.e. it increases the meter's K-factor (the number of pulsesper total volume of fluid flowing through the meter). The leakage effecthas a dependency on the fluid properties. In oscillating discs, thermalshocks can lead to serious distortions and damage to the disc.

In trying to eliminate the ports and the leakage passage used withshuttle elements, several proposals call for using diaphragms to sealthe cavity of the shuttle element. A sealed shuttle element allows, inaddition to proximity transducers, the use of a wider variety of straintransducers. Thin diaphragms are susceptible to thermal and pressureshocks. Often, an oil-filled cavity is used to support the diaphragmswithout serious damage to the response time of the sensor. Thermalexpansion and phase change conditions limit the application range of anoil-filled cavity. In both the cases of an oil or atmospheric air filledcavity, a damaged diaphragm can lead to releasing the flow fluid to theoutside environment. With few exceptions, such as shown in U.S. Pat. No.4,475,405, the replacement of the transducer requires depressurizing theflow line.

More recently, a proposal described in U.S. Pat. No. 4,625,564 calls forusing a fin in the passageway. The deflection of the fin activatesstrain transducers.

Other proposals describe techniques to measure the integrated pressuredepression along the side areas of the strut. This results inalternating lift and drag forces. The pivoting of the strut isolateseither a bending force or a torsional torque from a shearing force. Theintegration of the stress along the strut increases the complexity ofthe design, since the noise generated anywhere along the strut needs tobe eliminated. U.S. Pat. No. 4,437,350 outlines the use of piezoelectricelements to sense the minute strains due to the micro-bending of thepivoted strut. The piezoelectric elements are embedded (e.g. cemented)within the strut itself. If the piezoelectric elements fails the wholestrut must be replaced. The removal of the strut requires a flow shutdown. The clamping of the element is not mechanical. This makes thearrangement more expensive and more susceptible to loss of signal if thecement softens, such as might occur at higher temperatures. U.S. Pat.No. 4,699,012 describes a vortex sensing member disposed downstream ofthe vortex generator and parallel to it. The sensing member has aslender midsection to sense the lift forces downstream of the vortexgenerator. Different end support arrangements are proposed to suppressnoise and amplify the vortex shedding effect.

In an alternative approach, the alternating drag forces are used toexert a micro-twist upon a torque tube located downstream of a primaryshedder. The strains are transmitted through a link to piezoelectricsensing elements outside the flow.

There are other known techniques capable of detecting the vorticesdownstream of a shedder such as those which use ultrasonic transducers.

There is a need for an inexpensive and simple design for a differentialpressure transducer for use with a vortex flowmeter. Local measurementsrather than integrated ones are desirable since they tend to be lessnoisy. A mechanically clamped transducer design is simpler and moreserviceable than welded or cemented ones. A non-welded sensor that canbe replaced without depressurizing the flow line possesses a clearadvantage. The elimination of all leakage passages and ports shown insome types of prior art devices not only would avoid clogging of thesepassages with debris, but it also would result in a more linear outputindependent of the fluid properties. Another important property of asensor for a vortex flowmeter is the ability to reject common-modenoise, i.e. vibrations due to sources other than the alternatingvortices shed by the bluff body or strut.

SUMMARY OF THE INVENTION

A differential pressure sensor for a vortex flowmeter with a highcommon-mode rejection relies on sensing the stresses on a thinnedportion of a shedding body. The body is T-shaped with the head of the Tarranged perpendicular to the direction of fluid flowing in a conduit.The body is formed by machining or casting and includes a pair ofthinned circular areas or recesses which are arranged on opposite sidesof the upright portion of the T to receive the impact of vertices shedby the body. The machined recessed in the body are superior to prior artwelded thin diaphragms in resisting thermal and pressure shocks.

The body is placed in fluid flowing in a conduit with the two sides ofthe upright thin portion of the T under the static line pressure. Thedifferential pressure at the thinned portion is transmitted to a cavityformed at one end of the body and disposed outside the conduit. A pairof strain transducers are arranged in contact with a wall of the cavityand on opposite sides of a plane containing the upright portion of theT-shaped body. The strain transducers are responsive to flexing inducedin the cavity wall caused by impacts of vortices shed by the bodyagainst the thinned portion of the body.

The use of two transducers mechanically coupled to opposite sides of thebody reduces the sensitivity of the transducers to common-modevibrations. The transducers sense the minute out-of-phase strains on thetwo sides of the body. The common mode noise that results from pump orline vibrations affects the two transducers simultaneously in phase. Thesubtraction of the output of the two transducers amplifies theout-of-phase differential shedding signal and the cancellation of thecommon mode noise. This differential signal is a series of pulses whosefrequency is proportional to the velocity of the fluid flowing in theconduit. The pulses may be counted to give a visual indication of theflow rate, after appropriate scaling.

In one embodiment, the strain transducers are two piezoelectric elementswhich are mechanically clamped adjacent to or into contact with the wallof the cavity. The mechanical clamping means may include means forelectrically connecting the piezoelectric elements to a differentialdetection circuit. In a second embodiment the strain transducers are twoor more optical fibers having reflective ends adjacent to or in contactwith the cavity wall. Movement of the cavity wall causes a beam of lightwhich is guided through the fibers to be displaced (i.e. not reflected).The light beam is thus periodically reflected back along the axis of thefiber. These perodic alternations of the intensity of the reflected beamcan be detected by a photodetector and turned into a series ofelectrical pulses which can be counted and displayed.

Other strain transducers, like micro-bending optical fibers, capacitiveand resistive strain gages and the like, can be used.

The foregoing arrangements enable the vortex sensing elements to belocated in a dry area outside the fluid flow conduit. The vortex sensingelements can be readily removed and changed without a flow shut down.The shape of the vortex generator (shedding body or strut) is optimizedto generate vortices with enough energy to excite the vortex sensingelements and maintain a high degree of linearity and repeatability ofthe sensing of the vortices.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

These and other features and advantages of the present invention will bemore clearly understood from the following detailed description of thepreferred embodiments of the invention when taken with the accompanyingdrawing figures wherein:

FIG. 1 is a broken perspective view of a vortex flowmeter anddifferential transducer constructed in accordance with a firstenrollment the present invention:

FIG. 2 is a cross-sectional view of the flowmeter and transducer of FIG.1 taken along lines 2--2;

FIG. 3 is a cross-sectional view of the flowmeter and transducer of FIG.1 taken along lines 3--3;

FIGS. 4a and 4b and are top cross-sectional plan views of twoalternative shapes a shedding body that can be used with the flowmeterof FIG. 1;

FIG. 5 is similar to FIG. 2 but shows an alternative transducerarrangement utilizing fiber optic sensors;

FIG. 6 schematically shows an alternative optical fiber transducerarrangement similar to FIG. 5;

FIG. 7 schematically shows yet another alternative optical fibertransducer arrangement similar to FIG. 5; and

FIG. 8 shows one type of circuit which can be used with the transducersof FIGS. 2, 3, 5, 6, or 7 for providing a flowrate measurement, FIG 8ashows the signals output by the transducers, and FIG. 8b shows thesignal output by the circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1, 2 and 3, a vortex flowmeter 1 according to a firstembodiment of the invention comprises a conduit 3 having a central bore5 through which a fluid 7 flows in the direction indicated by arrow A. Aseries of flow rectifying vanes 9, which are described in more detail incopending application Ser. No. 129,122 filed December 4, 1987, arelocated at the entrance 11 of conduit 3.

A vortex shedding body or strut 13 having a generally T-shapedcross-section when viewed from above (see FIGS. 4a and 4b) is arrangedwith its long axis perpendicular to the direction A of fluid flowthrough conduit 3. The shape of body 13 is shown more clearly in FIGS.4a and 4b which represent two types of shedding body shapes which can beutilized to provide proper generation of vortices resulting from theflow of fluid 7 around body 13.

The head 15 of T-shaped body 13 is arranged substantially perpendicularto the direction A of fluid flow in conduit 3. The head 15 of body 13thus presents a blunt or non-streamlined surface which causes thegeneration of vortices downstream of the body in a well-known manner.

The upright portion 17 of body 13 takes the form of a rib joining head15 at substantially right-angles along the length of the body. Uprightportion 17 of body 13 is thus arranged substantially parallel to thedirection A of fluid flowing in the conduit but downstream of the head.

Upright portion 17 further includes a pair of circular recesses 19a and19b formed on opposite sides of the rib and at one end thereof. Thisresults in a relatively thin area (with respect to the normal thicknessof portion 17) between recesses 19a and 19b against which vorticescaused by shedding from the surfaces of head portion 15 of body 13 mayimpact. The impact of these vortices is subsequently detected, in amanner to be described below.

Except for the provision of the relatively thin portion resulting fromrecesses 19a and 19b, the shape of body 13 which is exposed to fluid 7may be conventional in design and may be formed from various materials,such as metals or plastics, in accordance with known principles.However, for best results, the shedding body shape should be optimizedin relation with the sensing technique such that the released vorticeshave enough energy to excite the transducer. The linearity and therepeatability of the flowmeter output depends mainly on the geometry ofthe body and the design of the transducer. These two factors (body shapeand transducer arrangement) are closely related and have to be optimizedtogether. FIG. 4a shows one type of optimized shedding body shape. Thefront face 15 of the body 13 is shaped such that the frontal stagnationline of the approaching flow stream 7 becomes fixed with a minimumjitter. The small flat sides 16 are provided to control the width of thefront face and ensure a relatively sharp edge 18 on an investment castbody.

Referring again to FIG. 1, 2, and 3, at one end of body 13 there isformed a cavity portion 21 with a wall 23 arranged approximatelyparallel to the direction A of fluid flowing in conduit 3. Cavity 21further includes a circular wall 24 surrounding wall 23, which togetherform cavity 21.

Cavity portion 21 of body 13 fits within an opening 25 formed along aradius of conduit 3. The opposite end of body 13 includes a portion 27which fits into a recess 28 formed in the wall of conduit 3 opposite tothat of opening 25.

Body 13 is secured to conduit 3 by means of plate 29 which fits overcavity portion 21 of the body. Plate 29 is secured to conduit 3 by fourthreaded fasteners 31. Gasketing material 33 may be interposed betweenthe outer portion of cavity 21 and the surface of conduit 3 where thetwo contact so as to provide a fluid-tight seal therebetween.

Wall 23 is arranged to receive a pair of strain transducers 35a and 35bwhich are arranged on opposite sides of a plane containing uprightportion 17 of body 13.

The proximity of wall 23 to recesses 19a and 19b causes wall 23 to besubject to flexing due to the impact of vortices shed by head portion 15of body 13 against recesses 19a and 19b. More particularly, adifferential pressure is developed between recesses 19a and 19b due topressure differences caused by the passage of vortices by recesses 19aand 19b. These pressure differences cause minute stresses in the portionof upright portion 17 between recesses 19a and 19b and wall 23 of cavityportion 21. These stresses, in turn, are transmitted to wall 23 and takethe form of a series of variations in surface stress acting alternatelyon opposite sides of a plane drawn through upright portion 17 of body 13and bisecting wall 23.

The stresses induced in wall 23 may be detected utilizing straintransducers 35a and 35b of conventional design. Preferably, however,transducer 35a and 35b are piezoelectric transducers or fiber opticreflective transducers, as discussed below.

Referring to FIGS. 1, 2 and 3 a transducer can be assembled usingpiezoelectric elements 35a and 35b and mechanical clamping. At thebottom of cavity portion 21, two piezoelectric elements 35a and 35b areproduced from one piezoelectric disc by splitting it into two halves.The two halves are separated from each other by an electrical insulator39. Two electrodes 41a and 41b are in contact with the top surfaces ofthe piezoelectric elements that have the same polarity. The twoelectrodes are mounted on an electric insulator 43. A flat plate 45 withtwo guide pins 47a and 47b helps apply the clamping pressure on thepiezoelectric elements more uniformly without applying a shearing stresson them. The cavity is kept dry with the help of a seal 49.

The output of piezoelectric elements 35a and 35b is a series ofelectrical pulses having a frequency directly related to the flow rateof the fluid in conduit 3.

FIG. 5 shows an alternative transducer arrangement utilizing a pair ofreflective optical fibers. More particularly, a pair of optical fibers135a and 135b are held within cavity 21 by means of fiber guide 137which fits within cavity 21. The ends 136a and 136b of optical fibers135a and 135b are reflective and are located firmly by fiber guide 137adjacent to wall 23 on opposite sides of a plane containing uprightportion 17 of body 13 and bisecting wall 23. Ends 136a and 136b of theoptical fibers are pressed against wall 23 by means of bushing 139 whichpresses against guide 137.

A pair of light emitters 138a, 138b, and light detectors 140a, 140b areassociated with the opposite ends of optical fibers 135a and 135b, asshown in FIG. 5.

Normally, light emitted by a light emitter 138a 138b is transmitted downits associated optical fiber, 135a or 135b, and reflected at ends 136aor 136b along the axis of the fiber back to light detectors 140a or140b, respectively. However, flexing of wall 23 induced by impacts ofvortices shed by body 13 against recessed areas 19a and 19b causes thereflective ends 136a and 136b to be offset slightly from their normalperpendicular relationship to the axis of their associated opticalfibers. This results in a periodic variation in the light intensityreflected back to light detectors 140a and 140b. Light detectors 140aand 140b, which for example are photodiodes, produce an electricaloutput which is a series of time varying electrical signals or pulseshaving a frequency directly related to the flow rate of the fluid inconduit 3.

FIGS. 6 and 7 show alternative embodiments of the optical fibertransducer shown in FIG. 5. For the sake of clarity, the optical fibers,light emitters, light detectors and wall 23 of the cavity portion 21 ofbody 13 are shown only schematically. In FIG. 6, two pairs of opticalfibers 235,237 and 239,241 are utilized for transmitting light emittedfrom light emitters 238a and 238b and for receiving light reflected fromwall 23 (which may include one or more reflective portions 223 thereon)by means of light detectors 240a and 240b. Light emitters 238a, 238b andlight detectors 240a and 240b are similar in structure and function tolight emitters and detectors 138a, 138b and 140a, 140b described abovewith respect to FIG. 5.

FIG. 7 shows yet another version of an optical fiber transducerutilizing a pair of optical fibers 335a and 335b, a single light emitter338, a first wavelength splitter 339, a pair of optical input fibers341a and 341b, reflective portions 323a and 323b formed on wall 23, apair of optical output fibers 343a and 343b, a beam combiner 345, asecond wavelength splitter 347, and a pair of light detectors 340a and340b. Light emitter 338 emits light of more than one frequency and istransmitted over optical fiber 335a to wavelength splitter 339.Wavelength splitter 339 splits the transmitted light into two wavelengthgroups, with one group being transmitted via optical input fiber 341a toreflective portion 323a of wall 23 and the other group being transmittedvia optical input fiber 341b to reflective portion 323b. Light reflectedfrom reflective portions 323a and 323b is transmitted via optical outputfibers 343a and 343b, respectively, to beam combiner 345 which acts likewavelength splitter 339, but in reverse. The combined light beam istransmitted via optical fiber 335b to wavelength splitter 347 whichseparates the two beams again and applies them to respective lightdetectors 340a and 340b. The signals from light detectors 340a and 340bare processed similarly to that described above with respect to FIG. 5.

FIG. 8 shows an arrangement for sensing the outputs A and B oftransducers 35a and 35b of FIGS. 1, 2 and 3, or the optical transducersshown in FIGS. 5-7. A circuit 51 for sensing the outputs of thetransducers is connected by wires 53 and 55 to electrodes 41a and 41brespectively. Circuit 51 may be housed in housing 57 (see FIG. 1) orlocated remote from flowmeter 1 and consists essentially of a differenceamplifier 51a which outputs a signal C whenever there is difference (Δ)between the signals at its input. Since the transducers are normallysubject to stresses of opposite phase but of similar magnitude (due tothe impact of vortices on body 13 transmitted to cavity wall 23), theoutputs A and B of the transducers will be of opposite sign but ofsimilar magnitude. Applied to the inputs of circuit 51, this results inan output signal C which is the difference (Δ) between the two inputsignals. This output signal C has essentially double the magnitude ofany one of the signals A or B produced by a transducer. The in-phasenoise drops out due to the subtraction performed by difference amplifier51a, as shown in FIG. 8b. Furthermore, the signal output by circuit 51takes the form of a series of pulses (see FIG. 8) since the outputs ofthe transducers are a series of approximately sinusoidal waves ofapproximately equal magnitude but 180 degrees out of phase with eachother, as shown in FIG. 8a. These pulses may be counted by a counter 51bto determine their frequency over a predetermined time period. The pulsefrequency is directly proportional to the velocity (flowrate) of thefluid flowing in conduit 3. The pulses output by circuit 51 may be usedto drive a counter or other display of conventional design disposed inhousing 57.

The circuit of FIG. 8 may be utilized with the reflective optical fiberarrangement of FIGS. 5, 6 or 7 by sensing the outputs A and B of lightdetectors 140a and 140b (or 240a, 240b, or 340a, 340b) since theyproduce output signals similar to those output by transducers 35a and35b of FIGS. 1, 2 and 3. These output signals are processed by circuit51 in the same fashion as described above with respect to FIG. 8.

It will be appreciated that, because of the arrangement of body 13,recesses 19a and 19b, cavity 21 and wall 23, the outputs of transducers35a and 35b (or the optical transducers shown in FIGS. 5, 6, or 7) areaffected in an opposite sense only when there is a difference inpressure against upright portion 17 of body 13 caused by the impact ofvortices shed by body 13. This means that the vortex transducers willexhibit very high common mode rejection. That is, stresses caused byexternal vibrations or forces acting on the conduit or transducer, willeffect both transducers equally and in phase. The outputs of thetransducers will therefore be in phase and of similar amplitude. Sincecircuit 51 is responsive only to differences in the outputs of thetransducers, signals due to common mode vibrations will result in nooutput from circuit 51.

The two described transducer arrangements (piezoelectric and opticalfibers) give high signal levels for both liquid and gas flows. Otherways based on the flexure of an optical fiber (like microbending,speckle pattern, etc.) may also be used to produce modulated signalsthat can be subtracted to cancel common mode signals in the same waydescribed above.

A further advantage of the invention is that the transducers may beeasily removed and/or replaced without the need to shut down fluidflowing in conduit 3. This is because the transducers are located incavity 21 which is on the "dry" side of conduit 3, i.e. outside the bore5 where fluid 7 flows. Also, the transducers are not permanently sealedwithin cavity 21 but only clamped, making removal of the transducers forrepair or replacement easy. Furthermore the shedding body is designedwithout any openings where debris may accumulate and is rugged in shapeand design. The shedding body may be removed from conduit 3 throughopening 25 if necessary for maintenance or inspection.

While the present invention has been described in considerable detail,it will be understood that various modifications and alternatives wouldoccur to those skilled in the art. Accordingly, the foregoing isintended to be descriptive but not limitive of the invention which isdefined by the following claims.

What is claimed is:
 1. A vortex flowmeter comprising:a conduit forconfining a fluid flowing therein; a vortex shedding body having aportion placed within the conduit for generating vortices from fluidflowing past the body; the body including a recessed portion having asurface which is subject to impact from the vortices generated by fluidflowing past the body; the body further including a cavity portionlocated at one end thereof and outside the conduit, the cavity having awall subject to alternate flexing caused by forces transmitted from thesurface of the recessed portion of the body to the cavity portion of thebody due to the impact of the vortices on the surface of the recessedportion of the body; and means for sensing the flexing of the wall ofthe cavity including; (a) at least one light source; (b) light detectionmeans; (c) reflective means disposed on the wall portion of the cavityand; (d) optical fiber means for transmitting light from the lightsource to the reflective means and to the light detection means; theflexing of the wall of the cavity causing an output from the sensingmeans which represents the difference between the amplitude of thealternate flexing of the wall caused by impact of the vortices generatedby the body.
 2. The flowmeter of claim 1 wherein the body is unitary andthe cavity and recessed portion of the body are formed by machining ofthe body.
 3. The flowmeter of claim 1 wherein the conduit includes anopening formed along a radius thereof for receiving the cavity portionof the body and wherein the recessed portion of the body is arrangedwithin the conduit and in contact with the fluid.
 4. The flowmeter ofclaim 1 wherein the portion of the body within the conduit has agenerally T-shaped cross-section in a plane parallel to the direction offluid flow in the conduit, with the head of said T-shaped sectionarranged substantially perpendicular to the direction of fluid flow inthe conduit to present a blunt surface thereto and the upright portionof said T-shaped section arranged substantially parallel to thedirection of fluid flow in the conduit.
 5. The flowmeter of claim 4wherein said upright portion of said T-shaped section includes twocircular recessed sections formed on opposite sides of the uprightportion and proximate the wall of the cavity portion.
 6. The flowmeterof claim 1, including means for detecting the alternating electricaloutput of the light detection means and for producing a signal which isthe difference between the amplitude of the signal output by each lightdetection means and wherein the frequency of the difference signal isproportional to the velocity of fluid flowing in the conduit past thebody, whereby detection of common mode vibrations induced in the body isminimized.
 7. The flowmeter of claim 1 wherein the sensing meansincludes a pair of light sources and a pair of light detectors, a firstlight source and light detector being coupled to one of a pair ofreflective portions provided on the wall portion of the cavity by meansof a pair of optical fibers and a second light source and light detectorbeing coupled to a second of the pair of reflective portions by means ofa second pair of optical fibers, whereby the transmission of light fromthe light sources to the light detectors is affected by the alternateflexing of the cavity wall and produces an alternating electrical outputof the light detectors, the frequency of the alternating electricaloutput being proportional to the velocity of the fluid flowing in theconduit past the body.
 8. The flowmeter of claim 1 wherein the sensingmeans includes one light source and a pair of light detectors, the lightsource emitting light of more than one wavelength, wherein the lightfrom the light source is transmitted over an optical fiber to awavelength splitter disposed proximate the cavity portion of the body,the wavelength splitter splitting the light into two beams which in turnare transmitted via a pair of optical input fibers to respective ones ofa pair of reflective portions provided on the wall portion of thecavity, the light reflected from the reflective portions beingtransmitted to a beam combiner disposed proximate the cavity of the bodyvia a pair of optical output fibers, light transmitted by the opticaloutput fibers being combined in the beam combiner and transmitted over asecond optical fiber to another wavelength splitter disposed proximatethe light source and light detectors, this other wavelength splittersplitting the light into two beams which are detected by the lightdetectors, whereby the transmission of light from the light source tothe light detectors is affected by the alternate flexing of the cavitywall and produces an alternating electrical output of the lightdetectors, the frequency of the alternating electrical output beingproportional to the velocity of the fluid flowing in the conduit pastthe body.
 9. A vortex flowmeter comprising:a conduit for confining afluid flowing therein; a vortex shedding body having a portion placedwithin the conduit for generating vortices from fluid flowing past thebody; the body including a recessed portion which is subject to impactfrom the vortices generated by fluid past the body; the body furtherincluding a cavity portion located at one end thereof and outside theconduit, the cavity having a wall subject to alternate flexing caused bythe impact of the vortices on the recessed portion of the body; meansfor sensing the flexing of the wall of the cavity including: (a) atleast one light source; (b) light detection means; (c) reflective meansdisposed on the wall portion of the cavity; and (d) optical fiber meansfor transmitted light from the light source to the reflective means andthen to the light detection means; and means for detecting analternating electrical output of the light detection means and forproducing a signal which is the difference between the amplitude of thesignal output by the light detection means and wherein the frequency ofthe difference signal is proportional to the velocity of fluid flowingin the conduit past the body, whereby detection of common modevibrations induced in the body is minimized.
 10. A vortex flowmetercomprising:a conduit for confining a fluid flowing therein; a vortexshedding body having a portion placed within the conduit for generatingvortices from fluid flowing past the body; the body including a recessedportion which is subject to impact from the vortices generated by fluidflowing past the body; the body further including a cavity portionlocated at one end thereof and outside the conduit, the cavity having awall subject to alternate flexing caused by the impact of the vorticeson the recessed portion of the body; and means for sensing the flexingof the wall of the cavity including: a pair of light sources and a pairof light detectors, a first light source and light detector beingcoupled to one of a pair of reflective portions provided on the wallportion of the cavity by means of a pair of optical fibers and a secondlight source and light detector being coupled to a second of the pair ofreflective portions by means of a second pair of optical fibers, wherebythe transmission of light from the light sources to the light detectorsis affected by the alternate flexing of the cavity wall and produces analternating electrical output of the light detectors, the frequency ofthe alternating electrical output being proportional to the velocity ofthe fluid flowing in the conduit past the body.
 11. A vortex flowmetercomprising:a conduit for confining a fluid flowing therein; a vortexshedding body having a portion placed within the conduit for generatingvortices from fluid flowing past the body; the body including a recessedportion which is subject to impact from the vortices generated by fluidflowing past the body; the body further including a cavity portionlocated at one end thereof and outside the conduit, the cavity having awall subject to alternate flexing caused by the impact of the vorticeson the recessed portion of the body; and means for sensing the flexingof the wall of the cavity including: one light source and a pair oflight detectors, the light source emitting light of more than onewavelength, wherein the light from the light source is transmitted overan optical fiber to a wavelength splitter disposed proximate the cavityportion of the body, the wavelength splitter splitting the light intotwo beams which in turn are transmitted via a pair of optical inputfibers to respective ones of a pair of reflective portions provided onthe wall portion of the cavity, the light reflected from the reflectiveportions being transmitted to a beam combiner disposed proximate thecavity of the body via a pair of optical output fibers, lighttransmitted by the optical output fibers being combined in the beamcombiner and transmitted over a second optical fiber to anotherwavelength splitter disposed proximate the light source and lightdetectors, this other wavelength splitter splitting the light into twobeams which are detected by the light detectors, whereby thetransmission of light from the light source to the light detectors isaffected by the alternate flexing of the cavity wall and produces analternating electrical output of the light detectors, the frequency ofthe alternating electrical output being proportional to the velocity ofthe fluid flowing in the conduit past the body.
 12. The flowmeter of anyof claims 9-11 wherein the body is unitary and the cavity and recessedportion of the body are formed by machining of the body.
 13. Theflowmeter of any one of claims 9-11 wherein the conduit includes anopening formed along a radius thereof for receiving the cavity portionof the body and wherein the recessed portion of the body is arrangedwithin the conduit and in contact with the fluid.
 14. The flowmeter ofany one of claims 9-11 wherein the portion of the body within theconduit has a generally T-shaped cross-section in a plane parallel tothe direction of fluid flow in the conduit, with the head of saidT-shaped section arranged substantially perpendicular to the directionof fluid flow in the conduit to present a blunt surface thereto and theupright portion of said T-shaped section arranged substantially parallelto the direction of fluid flow in the conduit.
 15. The flowmeter ofclaim 14 wherein said upright portion of said T-shaped section includestwo circular recessed sections formed on opposite sides of the uprightportion and proximate the wall of the cavity portion.